It is well known that the delivery of materials to the body can be via syringes and hypodermic needles but such devices are generally used to deliver a relatively large amount of material at a defined point in time as a single dose. More recently the medical profession has seen the advantage of delivering a smaller amount of material over a time period and to do this transdermal delivery devices have been developed which have an array of microneedles that are in contact with the skin. The microneedles deliver a small amount of material over a period of time and to do this the microneedles have smaller diameters and sharper tips than conventional needles in order to minimize pain and damage to the skin. The microneedles could also be used for blood and cell sampling.
Microneedles have been fabricated from a range of materials including metals, silicon, silicon dioxide, polymers and glass. The mechanism for drug delivery using microneedles is not based on diffusion as is the case in traditional hypodermic delivery but on the mechanical disruption of the skin and the placement of the drug within the epidermis which has capillaries that can then take up the drug and deliver it to the site of action in the body. The microneedles have a length of from a few hundred micrometers to a few millimeters so that they only penetrate the superficial layers of the skin where the density of nerve receptors is low and consequently the use of microneedles is perceived as a being a painless way of delivering drugs, vaccines, and/or cells.
Microneedles are generally produced from a substrate and in the case of silicon needles the needles are produced from wafer of silicon. Thousands of needles can be fabricated on a single wafer and this leads to the production of needles with a high degree of reliability and accuracy.
It is known to fabricate silicon microneedles with flat tips by dry etching technologies using various gas compositions and processes. However, dry etching processes are expensive, and the process does suffer from some drawbacks including for example the requirement for specialised equipment operated by highly trained personnel. Also, batch processing is generally not possible as usually only one wafer can be treated at a time.
U.S. Pat. No. 5,855,801 describes a wet etch process using hydrofluoric acid for fabricating microneedles by providing a substrate, depositing an un-etchable membrane over the substrate, and opening etching holes in the membrane layer to allow flow of etchant underneath to form a cavity and provide a desired needle shape.
However both processes have the disadvantage that if a dry etch method if used then a high level of control is needed to obtain consistency and uniformity in the shape of the needles that are produced, which make the process expensive. In the case of wet etch processes again because the etching process cuts across several crystal planes of the material that is being etched then precise control of the process is needed which may not be compatible with the dry etch processes.
The invention seeks to overcome the prior art by providing an improved process for fabricating microstructures in a consistent and cost effective way.